It is critical that industrial electrical food heating and cooking devices such as ovens and fryers operate efficiently and reliably under nearly continuous usage over long time periods. Many such devices in the past operated at relatively low power levels, so that even non-optimum internal electrical connection arrangements did not present significant problems. In many industrial applications (e.g., preparation of precooked food products), however, heating devices which can elevate the temperature of heating chambers rapidly are required. Such heating devices may draw large amounts of electrical power (e.g., 10 kilowatts per heating element array). Unless industrial heating device electrical connections are properly designed, such large current levels can cause the connections to fail in a variety of modes, some of which present hazards to human life.
FIG. 1 is a schematic diagram of a typical "wye" connected oven heater element matrix 50 for connection to a three-phase, 450 VAC power source.
Heater element matrix 50 includes three heater element sub-matrices 52a, 52b and 52c, each sub-matrix being connected to a different phase of the 3-phase power source. Each sub-matrix 52 includes two columns 54 of heater elements, each column including five individual heater elements 56. All elements 56 are connected between common neutral bus bar 58 and a "hot" phase terminal 60 (one "hot" terminal being connected to each of the three incoming power line phases).
In the prior art arrangement shown in FIG. 1, each phase terminal 60a-60c is connected to ten parallel heating elements 56, and each heating element has a 100 watt power rating. The nominal voltage between each phase terminal 60a-60c and neutral bus 58 is 277 VAC, and each heating element 56 draws about 3.6 amps. Accordingly, each phase terminal 60a-60c supplies about 36 amps at 277 volts--or about 10,000 watts.
FIG. 2 is a perspective view of a prior art heater element matrix 50 corresponding to the FIG. 1 electrical connection diagram. In the FIG. 2 arrangement, electrical connections are established between phase terminals 60a-60c and heater elements 56 (the heater elements being hidden from view inside housing 65 shown in FIG. 2). An array or grid of parallel strip conductors 70 connects phase terminals 60a-60c and neutral bus bar 58 to respective heater element terminals 57.
FIG. 2A shows a prior art oven 20 of the type adapted to use heater element matrices 50. Oven 20 includes a hood 22 defining a chamber therein. A moving belt for supporting items to be heated is disposed within the chamber. In the embodiment shown, four banks or "zones" 24(1)-24(4) of six heater element matrices 50 each are installed in hood 22 to heat the air within the chamber to very high temperatures. Electrical connection boxes 26(1)-26(4) are each supplied with incoming power line "phases" from a main controller (not shown) of conventional design via conduits 28(1)-28(4). Further conduits 30(1)-30(4) enclose insulated cables which connect connection boxes 26(1)-26(4) to branch panel boxes 32(1)-32(4). Branch panel boxes 32(1)-32(4) in turn supply three phase power to each of the matrices 50 in banks 24(1)-24(4), respectively. Boxes 26 and 32, conduits 28 and 30 and matrix enclosures 65 are preferably made of conductive material (e.g., steel or aluminum) and are mutually connected together and to a common grounding point--- all as well known to those skilled in this art. Enclosures 65 each include a cover 34 which protects operators from shock and exposure to the electrical connections within.
FIG. 2B is a perspective view of a heater element matrix 50. In the embodiment shown, heater elements 56 are elongated tubular structures having a loop portion 56a shaped as a letter "U" and terminating in two end portions 56b, 56c. These conventional heater elements 56 include an outer tubular shell enclosing a drawn high resistance conductor which insulative material prevents from contacting the shell. When AC current is connected across element 56, the element internal high resistance conductor emits heat which is then radiated by the element tubular shell. Element end portions 56b, 56c terminate in the terminals 57 shown in FIG. 2.
FIG. 3 shows a more detailed view of a single connection strip 70 and the manner in which that connection strip is connected to five heater element terminals 57a-57e.
In the prior art arrangement shown in FIG. 3, each heater element terminal 57 includes an insulating spacer 74 having a longitudinal cylindrical passage formed axially therethrough, and a threaded cylindrical post 76 passing through the spacer passage. The portion of threaded post 76 protruding from the top of spacer 74 is used as a connection terminal post 72 to connect corresponding heater element 56 with connection strip 70. A conventional metallic nut 78 has threads which are engaged with the threads of post 72. Spacer 74 is preferably mounted on a flat supporting insulative surface (not shown), and the lower end of post 76 (as shown in FIG. 3) connects with (and mechanically supports) heater element 56.
Connection strip 70 is a conductive (e.g., nickel copper), relatively thin elongated flat bar having five holes 80 drilled therethrough at regular intervals along the length of the strip. Terminal posts 72 are passed through respective holes 80, and nuts 82 are threaded onto the posts. Nuts 82 are tightened until strip 70 is firmly held in place between nuts 78, 82--establishing an electrical connection between the strip and the opposing nuts. Some current may flow between the strip 70 and terminal post 72 via the threads of the post which happen to contact the strip, but most of the current flow is through the nuts which contact both the strip and the post. A conventional threaded locknut 84 is engaged with terminal post 72 above nut 82 and is tightened to prevent nut 82 from loosening.
The FIG. 2 connection matrix 50 suffers from the disadvantage that most or all of the current flow to/from heating elements 56 is through threads of nuts 78, 82, 84 which engage the threads of terminal posts 72. This arrangement is adequate only if the current drawn by the heater elements and the voltages applied to phase terminals 60 are relatively small. However, a modern industrial oven has a large heating chamber which requires a substantial amount of heating power to be rapidly elevated to operating temperature and to be maintained at nearly constant, high temperature levels despite variations in chamber heat loading. Consequently, arrangements such as those shown in FIGS. 2 and 3 have been (improperly) used in the past to provide 10,000 watts of energy from each of these phases of input AC voltage. These prior art arrangements experience relatively high failure rates when used continuously in production environments. Some of the failure modes these arrangements exhibit are potentially dangerous to human operators, and all failure modes increase production plant down time and add to plant maintenance and repair costs.
I have discovered that most of the failure modes of the FIG. 2 and 3 arrangements are caused by the threaded structures of heater element terminal posts 72. FIG. 4A is a detailed cross-sectional elevated view of an assembled FIG. 3 contact structure. When locknut 84 is tightened, the lower locknut surface 90 moves into close contact with nut 82 upper surface 92. However, because post threads 94 are formed in a spiral along post 72 (as virtually all conventional threads are formed), locknut lower surface 90 tends to be slightly skewed or tilted from a position exactly perpendicular to post axis 96.
Because of this slightly tilted orientation of locknut 84 (and the resulting unequal distribution of forces within the locknut and nut 82), locknut threads 98 do not precisely engage or mate with corresponding post threads 94 around the entire circumference of post 72. Rather, many of the locknut threads 98 actually float between corresponding post threads 98, decreasing the total surface area of locknut 84 in contact with post 72. Locknut threads 98 in the so-called "crimp" region 95 are in close contact with post threads 94 because of the upward forces resulting from close contact of locknut lower surface 90 with nut 82 upper surface 92. However, the total surface area of locknut threads 98 in this "crimp" region 95 may be (and generally is) insufficient to conduct the large current drawn by heater element 56.
Nut 82 threads 100 typically float between the corresponding post threads 94 and only minimally contact the post threads because nut 82 is suspended by the opposing forces exerted on it by locknut 84 and strip 70.
Conductive strip 70, not being itself threaded, typically does not directly contact post 72. Threads 102 of lower nut 78 contact post threads 94 at some thread surfaces, but because of the downward forces exerted on this lowermost nut by locknut 84 (through upper nut 82 and strip 70), there is rarely full contact between all surfaces of lower nut threads 102 and post threads 94.
FIG. 4B shows schematically how electrical current flows through the FIG. 4A electrical connection. Because strip 70 does not generally directly contact post 72, current cannot flow directly between the strip and the post but instead must flow through nuts 78, 82 and/or 84. Assuming no oxidation has yet occurred and the nuts 78, 82 and 84 are all tight, current can flow from strip 70 into nut 82 along path E1, and then flow from nut 82 into post 72 through nut 82 threads 100 along path C. Current can also flow from strip 70 into nut 82 along path E1, from there flow between contacting surfaces 90, 92 into locknut 84, and finally, flow from the locknut into post 72 along path A (through the locknut threads 98 in crimp region 95) and/or along path B (through locknut threads other than those in the crimp region). Current can also flow along path E2 between strip 70 and lower nut 78 surface 93, and from the lower nut into post 72 through lower nut threads 102 along path D.
Unfortunately, the total cross-sectional contact area between the nuts 78, 82, 84 and post 72 is relatively small. Only a portion of the total surface areas of nut threads 98, 100 and 102 are direct contact with post threads 94 -- substantially decreasing the cross-sectional area through which current may flow into (out of) post 72.
Because of this relatively limited cross-sectional area available for current flow, the electrical connection shown in FIGS. 4A and 4B commonly fails when used in industrial grade food ovens and fryers.
For example, most or all areas of nuts 84, 82 and 78 of the FIG. 4A connection which are not in direct contact with post 72 become badly oxidized. Typically, the entire surface areas of nut 82 threads 100 become badly oxidized (indicating that current cannot flow through those threads and post threads 94 and further preventing such current flow), and nut upper surface 92 likewise becomes fully oxidized. Locknut threads 98 typically also become at least partially oxidized due to inadequate close contact caused by nut tilt. Post threads 94 also generally become oxidized due to absence of good electrical contact with nut threads 98, 100 and 102.
Due to these phenomena, the FIG. 4A connection is inadequate for carrying the current required by heater elements 56. While not all FIG. 4A connections fail, a substantial percentage of them do fail in various, serious ways. For example, in a representative FIG. 3 assembly which I analyzed for failure, the following failure conditions were observed.
In the case of heater element 56a, the threads 100 and associated contact surfaces of nut 82a became completely oxidized due to floating of these nut threads between post threads 94, preventing current from flowing from post 72a into strip 70 through nut 82a along path C shown in FIG. 4B. Nuts 84a, 82a were still tight, but nut 78a has loosened--causing full oxidation of the upper surface 93 of this loosened nut 78a and the corresponding lower surface of strip 70. The conditions existing at heater terminal 56a led to current transfer to post 76 only along FIG. 4B path A through the threads of locknut 84 in the crimp region 95. Current from phase terminal connection bridge 87 passed into upper nut 82a but failed to transfer to post 72a through the threads 100 of the upper nut (path C) due to nut float. The current continued to flow into locknut 84a, but could only transfer into post 72 through threads 94 in the tight crimp area 95 (see FIG. 4A) -- all of the other threads of the locknut (i.e., those along FIG. 4B, path B) being heavily oxidized and/or burned. Due to the loosening of lower nut 78a and resulting oxidation of the surfaces of that nut and of the corresponding lower strip surface, no electrical contact existed between the bar and the lower nut along FIG. 4B path D.
In respect of the conduction to heater 56b, both sides of strip 70 were bright ringed (indicating current was still capable of flowing through both of nuts 78b, 82b). Locknut 78b was found to be loosened and its threads 102 were oxidized but apparently still capable of carrying current along FIG. 4B path D. Nut 82c upper surface 92 was heavily oxidized and burned, as were many of the threads 98 of locknut 84c--so the current was capable of flowing only through FIG. 4B path A and not along path B.
In the case of heater element 56c, both sides of strip 70 were found to be bright ringed, but transfer of current between nut 82c and locknut 84C was not possible because nut 82c upper surface 92 became fully oxidized. Current could flow only along FIG. 4B paths C and D, and not along paths A and B. Post threads 94 were somewhat oxidized but were still capable of carrying current.
Heater elements 56d and 56e has also failed or nearly failed, exhibiting conditions which were intermediate in severity to those of heater elements 56b and 56c.
In summary, it was found that the electrical connection shown in FIGS. 4A and 4B generally provided poor electrical conduction at the point of contact of between threads. Because locknut 84 is a crimp against upper nut 82, the locknut presses against the upper nut and applies a downward force on lower nut 78. The floating (disengaged) upper nut 82 threads 100 end up with poor, if any, electrical contact with post threads 94. If locknut 84 is crimped only to upper nut 82 (as appears to be the case in some instances), lowermost nut 78 may be left floating, resulting in burning of the upper and lower surfaces of strip 70.
These failures which virtually always occur in the FIG. 2 heater connection matrix 50 after normal use are serious enough to require replacement of the entire heater unit. Strips 70 in the vicinity of heater posts 72 become burned, and in some cases, arc sputtered. Strips 70 also become heat oxidized and warped from excessive heating of nuts 84, 82 and 78 due to bad electrical connections between the nuts and the heater terminal posts 72. Strips 70 also became blued or browned (black scale conditions) from excessive heating due to arcing between post 72 and strip 70 and/or nuts 78, 82 and 84. All terminations evidence heavy oxidation with complete oxidation of threading except in locknut 84 crimp areas 95. As burn cones develop, the transfer of current moves from nuts 78 and 82 to only the crimped area of locknut threads 98. Locknuts 84 then melt in the region of crimp area 95 because an insufficient cross-sectional contact area is called upon to carry an excessive amount of current. Oxidized terminals begin to flash over to housing 65 due to the relatively high voltages on the terminals and insufficient clearances between the terminals and the housing (phase-to-neutral conductive clearances in the prior art FIG. 2 arrangement were found in some cases to be less than 0.25 inches)--causing further melting of terminals and further excessive heating and also dangerous arcing ground faults where 480 VAC wiring is arc connected to the main oven bodies..
Even the phase terminals 60 in the prior art FIG. 2 matrix 50 were found to break down. For example, nickel plated screws 120 typically burn out completely due to oxidization of the screw threads, causing nickel plated wires 122 to arc melt away from the terminal. As will be understood, loose wire 122 within housing 65 can present a life-threatening hazard in cases where grounding of the furnace housing is insufficient or has failed (and at the very least, may cause circuit breakers or other protective devices to open the heater circuit and shut down the entire oven).
The prior art FIG. 2 termination failures lead to excessive levels of load imbalance on oven feeder lines connected to phase terminals; 60 (over 20% or more) due to numerous cases of defective terminal connections. As the in service life of the FIG. 2 matrix is prolonged, more and more of heater elements 56 become inoperative until the entire matrix fails and must be replaced.
This prior art matrix 50 fails many other reliability and safety criteria related to electrical insulation and connection design. For example, the electrical insulation used is extremely deficient, especially considering the temperature of operation of industrial ovens and the effect high temperatures have on air ionization and reduction of the insulative properties of air. Frequent flashing over between the phase conductors and ground conductors is in evidence and life safety hazards are presented unless outer housings are well tied to ground conductors -- and at the very least, flashing over wastes energy, causes excessive currents to be drawn, presents fire hazards. These events force premature replacement or modification of the equipment, and frequently force electrical system disconnection due to protective device sensing of the ground fault occurrence. Connection overheating due to use of screw thread connections may give rise to equipment fire or main conductor meltdown, and eventually causes most of heating elements 56 to become inoperable or inefficient as has been described.